Metallurgical processes frequently produce by-products which are of small and non-uniform particle size. Such by-products often contain value metals and may be returned to extractive process steps for further metal recovery. The metals in by-products most frequently considered for recovery in a recycling step, are one or more of the following:
______________________________________ Copper Titanium Aluminium Zinc Manganese Germanium Nickel Niobium Zirconium Cobalt Molybdenum Precious & Noble Metals Chromium Cadmium Platinum Group Metals Tungsten Tin Vanadium Lead ______________________________________
Other metals and their compounds may also be considered for recovery, depending on their abundance in the by-product or on the nature and use of the particular metal.
The process of the present invention is directed to metallurgical by-products in which non-ferrous metals predominate. For the sake of clarity, copper, zinc, nickel, cobalt, chromium, molybdenum, manganese, cadmium are sometimes referred to as transition metals, and tungsten, vanadium, titanium, zirconium, vanadium, niobium, molybdenum and tantalum are sometimes referred to as refractory metals. However, there is no clear division between these metals. Their recovery by recycling the dust or by-product which contains them, depends on usual economic considerations.
By-products, or waste products containing the metals to be recovered by recycling to extractive process steps may originate in various metallurgical installations, such as electrostatic precipitators and condensers. By-products may also be obtained as fumes, vapour deposited residues, dross, crushed spillage, and residues found in ladles and such like. Spent catalysts of various chemical processes, metal containing particles obtained from previously commercially used items which have valuable metallic components, such as for example, used catalysts of the automotive industry, are often returned to metallurgical processes to be re-extracted.
Particles entrained by slag are sometimes liberated by grinding. Value metals may also be retained in bricks of furnace lining, and may be separated by crushing such bricks. Slime and residue of electrolytic processes often contain metals which may be recovered by recycling to smelting process steps.
Various metal working processes may also yield waste products which may be a source of metals to be recovered by recycling.
The above are some of the more common sources of dust and small particles having non-uniform size range which may be recycled to metallurgical process steps for recovery. There may be other processes related to utilizing metals and providing metal particle-containing by-products or waste products, which then may be considered economically suitable for recycling to extractive metallurgical installations.
It will be clear to a person skilled in the art that there are many reasons why dust particles originating in metallurgical processes, or a by-product of such processes, should be prevented from being airborne, either during being charged to a furnace or during storage. It is the object of an agglomerating process to eliminate, or at least substantially reduce the probability of such particles of small size being easily airborne. It will also be apparent that the cost of agglomeration of the above discussed dust particles should be kept as low as possible.
In our U.S. Pat. No. 4,865,642, a process is described wherein particles originating in metallurgical processes may be agglomerated by mixing the dust with a combustible agglomerating agent. In that process the amount of combustible agglomerating agent added to the by-or waste product is related to the bulk specific gravity and to the particle size range of the dust to be agglomerated.
It has now been found that the process of agglomerating dust particles by mixing with a combustible agglomerating agent may be unexpectedly improved by mixing the dust particles with the combustible agent in the form of an aqueous emulsion, as distinct from being added in a fluid but undiluted state. It has been found that when utilizing an aqueous emulsion of the combustible agglomerating agent, a larger number of small particles may be coated and thereby agglomerated by the same amount of agglomerating agent, than it is possible when the agglomerating agent is brought to a fluid state by mere melting. It is to be noted, that the water added in the aqueous emulsion, is not permanently retained by the dust particles.
The amount of agglomerating agent required in the improved agglomerating process is reduced, resulting in reduced cost of agglomeration.
It is known to utilize a binder mixture, which contains amongst other components aqueous emulsions of hydrocarbonaceous substances, in briquetting ores, ore waste materials, blast furnace by-products, etc. It is to be noted, that briquetting is a process which applies a substantial force to the surface of the agglomerates, and that this force is applied substantially uniformly over the total surface of the agglomerate. If there is excess moisture or oil present in the mixture of dust and binder, such excess is usually squeezed out during briquetting and thereby removed. Moreover, the briquetting step is usually followed by an induration or heat treatment step, to allow the binder to form chemical bonds and thereby increase the strength of the briquettes. It is also to be noted that briquetting requires expensive equipment; thus briquetting and the subsequent heat treatment of the agglomerates may substantially increase the cost of the agglomeration process.
U.S. Pat. No. 2,828,325 issued to A. R. Subervie on Oct. 1, 1957, discloses a process for pelletizing, in a suitable press, agglomerates of dust and a binder mixture. The binder mixture of the process of the above patent comprises an aqueous emulsion of a hydrocarbonaceous material, ground solid pitch and an hydraulic binder. The role of the hydraulic binder in the binder mixture is to absorb and react with the water present in the agglomerates formed with the dust and the binder mixture. The requirement of having a hydraulic binder in this process increases the cost of waste product agglomeration. Furthermore, the subsequent feeding of the hydraulic binder containing pellets to a metallurgical furnacing operation is likely to increase the slag burden with the attendant economic effect.
U.S. Pat. No. 3,966,427, issued to R. Herment et al. on June 29, 1976, discloses a process directed to briquetting ore particles or wet cakes originating in a steel mill with a bituminous emulsion. The Herment et al. process requires bitumens which have a certain well-defined melting point range. The bitumen emulsion of U.S. Pat. No. 3,966,427 is stabilized by alkali metal hydroxide and resin additions, or in the instance of agglomerating an aqueous steel mud slurry, by the alkalinity of the water contained in the mud. Thus, the aqueous bitumen emulsions utilized require the presence of soaps or saponifiers, or control of the pH of the water present in the emulsion. Such additional process steps make the utilization of bituminous emulsion more costly. Furthermore, the agglomerates of mud particles mixed with the bitumen emulsion need to be compressed to form briquettes.
It is to be pointed out that in the prior art processes utilizing hydrocarbon-containing emulsions as briquetting agents, the waste product, or by-product particles to be briquetted, contain substantial amounts of iron compounds. It is well-known that iron and carbon under compression and reducing conditions combine to form a temporary binder. It is doubtful that agglomerating iron deficient dust particles with an aqueous bitumen emulsion may be achieved in the absence of an alkali emulsifying agent, and without the application of compacting by pressure.